remove monotonic as a dependency

"On Python 3.3 or newer, monotonic will be an alias of time.monotonic from the standard library"

app/__init__.py

@@ -23,7 +23,7 @@ from flask_login import LoginManager
 from flask_wtf import CsrfProtect
 from functools import partial
 
-from monotonic import monotonic
+from time import monotonic
 from notifications_python_client.errors import HTTPError
 from notifications_utils import logging
 from notifications_utils.recipients import validate_phone_number, InvalidPhoneError

requirements.txt

@@ -8,7 +8,6 @@ boto3==1.3.0
 Pygments==2.0.2
 py-gfm==0.1.2
 blinker==1.4
-monotonic==0.3
 lxml==3.6.0
 pyexcel==0.2.1
 pyexcel-io==0.1.0